The possibility of implementing an accurate real time dosimetry of elementary particle sources would be a remarkable progress in several important applications, such as radiotherapy.
For example, a branch of radiotherapy which would be significantly improved is brachytherapy. Coronary artery disease is the leading cause of morbidity and mortality in the western world. Re-establishing a stable and normal artery cross section (lumen) is the primary goal of angioplasty. On the other hand, re-narrowing of the cross section (restenosis) is the major limitation of angioplasty. Clinical studies indicate that intracoronary irradiation reduces substantially the problem of restenosis. It is estimated that the restenosis rate may drop from an original 30-40% below 10% if radiation is delivered to the obstruction site during or after angioplasty. The radiation treatment of an artery affected by restenosis (intravascular brachytherapy) is focused on a few centimeters long section of the vessel and it is usually accomplished either by multiple point-like radioactive sources placed on a catheter or by coil-shaped radioactive catheter tips or by wrapping with a radioactive foil the angioplasty balloon. During the radiation treatment the patient will have greatly reduced arterial blood flow, so, in order to reduce risks of complications, high dose rates are preferred; however, sources delivering dose rates of e.g. 5 Gy/min require monitoring of the activity uniformity.
Moreover, brachytherapy is characterized by steep dose gradients and three dimensional dose distributions, requiring high spatial resolution. A thorough understanding of the dosimetry of brachytherapy sources with high spatial resolution is important for addressing several key elements for the therapy optimization, including a full characterization of the interaction of radiation with vascular tissues, radiation penetration in different materials, dose profile inside the artery being treated and the definition of a treatment planning according to the vessel specifications. Moreover, a method and device for hospital based quality control of the sources would offer the possibility to improve the interventional safety conditions.
Real-time dosimetry could also improve safety conditions in oncological radiotherapy. Radiotherapic treatments based on X-rays are currently envisaged for 50% of patients affected by tumors. Among these, 30-40% are diagnosed as having a tumor or lesions that could benefit from irradiation with light ion beams. Approximately 250000 patients a year could benefit in Europe from a treatment with light ion beams; no better alternative exists for a sub-sample corresponding to 10% of the patients. The beam diagnostic systems of an hospital based accelerator for tumor radiotherapy is crucial as it determines an efficient and safe operation of the beam lines. A real time beam monitor could be based on the detection of electrons evaporated by the beam impinging on a thin target, appearing as an extended source of beta particles.
As of today, dosimetry, in particular of brachytherapy sources, is accomplished by two different classes of detectors with complementary features.
Passive detectors, namely radiochromic films not sensitive to visible light, feature a direct color change as a consequence of the energy deposition by elementary particles. The degree of environmental dependence of the radiochromic process can be accounted for in the calibration procedure and in the storage prescription; image scanning with a microdensitometer leads to a submillimetric resolution of the digitized image. The main drawback of radiochromic film dosimetry is the latency in the full image development; as a consequence of slow radiation induced chemical reactions, the American Association of Physicists in Medicine recommends the analysis of radiochromic films at least 24 h (preferably 48 h) after the exposure.
Active detectors feature a real time response to energy deposition by detecting scintillation light, gas ionization or through thermoluminescent effects. Calibration of the detector response guarantees the possibility to perform an accurate dosimetry in real time but the geometry of the detectors does not allow the reconstruction of accurate dose maps.
Only quite recently, customized CMOS imagers have been proven to be sensitive to charged particles and soft X-rays. The key element of existing CMOS particle detectors is the use of an n-well/p-epi diode, which is formed in an epitaxial layer grown on a substrate. More precisely, the diode collects through thermal diffusion the charge generated by particles impinging in the epitaxial layer.
However, in this case radiation tolerance should be improved, since known devices can be seriously damaged by high energy beams. In particular, thick oxide structures are required for insulating the n-well/p-epi diodes from the front-end circuitry. When exposed to a radiation beam, these thick oxide structures can capture charged particles, which are not subsequently released. Therefore, charge can be accumulated in the vicinity of the n-well/p-epi diodes, thus modifying electric field lines and efficiency is impaired.